Gas hydrate is a stable solid crystal under a low-temperature and a high-pressure condition in which gas molecules are contained in a cage-shape structure formed by water molecules.
Gas hydrate is capable of containing substantially 170 m3 gas molecules per a volume of 1 m3. Hence, gas hydrate is also studied and developed as a transporting and storing medium of natural gas (see Patent Literatures 1-3 and Non-Patent Literature 1).
In recent years, presentation of gas hydrate (hereinafter, referred to as natural gas hydrate) containing methane molecules as a primary component in permanently frozen grounds and deposit layers at sea bottoms have been confirmed (see Non-Patent Literature 1). In addition, a large amount of natural gas hydrate present in deposits in the sea around Japan has also been confirmed, and a development as a non-conventional energy resource to replace petroleum oil and coal is advancing.
An example researching method for natural gas hydrate is a so-called seismic method of causing vibrations (sonic waves) like earthquakes, and checking the reflection speed from deposit layers, and the distributed condition of natural gas hydrate present in the natural world has been researched through such a method.
In addition, excavation of deposit layers and inspection of the amount of resources and necessary physical properties for gas productivity evaluation using various sensors have been carried out. Still further, target deposit layers are collected, and the physical properties of the deposit sample of natural gas hydrate is researched. This enables a highly precise evaluation of the amount of resources and the gas productivity.
Several reports have been made for such methods of collecting deposit samples.
Patent Literature 4 discloses a scheme of performing sampling on, for example, pore water in deposits and microorganisms contained therein while maintaining the condition of the geological layer.
Meanwhile, gas hydrate is stably present under a low-temperature and a high-pressure condition, but when the condition becomes a temperature and a pressure condition out of the phase-stable temperature and pressure range of gas hydrate due to a temperature rise and a pressure reduction, hydrate will decompose. Hence, several schemes of collecting and analyzing a deposit sample containing gas hydrate so as not to allow gas hydrate decompose have been reported.
Non-Patent Literature 2 discloses, for a deposit sample collected with the temperature and the pressure being maintained, a method of performing a saturation factor measurement, a thermal conductivity measurement, a mechanistic responsiveness measurement of hydrate contained in the deposit sample, and of collecting the pore water in deposits. According to this method, with a deposit sample protected by a liner having undergone a coring being as a target, for example, an opening is formed in the liner to allow an analyzer device to contact the sample, thereby performing an analysis in a pressurized container (in general, the collected deposit is covered and protected by a plastic liner so as not to apply shock to the deposit at the time of coring). According to this method, after the sample is collected, evaluation of the physical properties of the deposit is enabled in the pressurized container without causing the gas hydrate in the deposit to decompose while maintaining the pressure.
In addition, Non-Patent Literature 3 discloses a method of collecting a deposit sample that has been collected with the temperature and the pressure being maintained, and producing a sample piece for analysis. According to this method, a gas hydrate deposit sample is pushed out as a cylindrical column in the axial direction, is cut in a desired length using a ball valve, and is kept in another pressurized container.
Still further, the ratio in volume between sands forming the deposit layer and materials other than sand (hereinafter, referred to as a porosity) is strongly associated with the permeability of gas and that of water, and an example method for measuring the porosity of the deposit is generally to first let the deposit dry, and then to measure the volume and weight of the dried deposit (hereinafter, referred to as a drying method). Since the deposit layer containing gas hydrate is formed of unconsolidated sands, however, according to the drying method of utilizing the dried sample that has gas hydrate decomposed, sands are moved due to the decomposition of hydrate, the layout of sand grains becomes different from the condition in which the sample was present in the geological layer, and the sample that contains more gas hydrate in the pores is more likely to change the volume since the layout of sands changes due to the produced decomposed gas. In addition to the drying method, a method of making the pores in a deposit visible through an X-ray computed tomography (CT) method and a synchrotron CT method, and calculating the porosity is also known (hereinafter, referred to as a CT method). According to conventional CT methods, however, since the deposit containing gas hydrate is once released to an ambient pressure, and the gas hydrate is thus decomposed, there is a problem that the layout of sands changes. As explained above, according to conventional drying and CT methods, precise evaluation of the gas productivity is quite difficult.
Hence, a method of measuring the porosity regardless of the conventional drying and CT methods has been reported. Non-Patent Literature 4 discloses to release a gas hydrate deposit having the pressure maintained to an ambient pressure, to quickly freeze the gas hydrate deposit using a liquid nitrogen, and to perform an X-ray CT imaging on the frozen sample, thereby measuring the porosity in a non-destructive manner.